Low Barri er RF Sch ottk y Dio d e BAT2 4 Mi xe r f or FMCW Ra dar at 2 4 GHz Application Note AN190 Revision: V1.0 Date: 22-01-2010 RF and Protecti on Devi c es Edition 15-02-2010 Published by Infineon Technologies AG 81726 Munich, Germany © 2010 Infineon Technologies AG All Rights Reserved. Legal Disclaimer The information given in this document shall in no event be regarded as a guarantee of conditions or characteristics. With respect to any examples or hints given herein, any typical values stated herein and/or any information regarding the application of the device, Infineon Technologies hereby disclaims any and all warranties and liabilities of any kind, including without limitation, warranties of non-infringement of intellectual property rights of any third party. Information For further information on technology, delivery terms and conditions and prices, please contact the nearest Infineon Technologies Office (www.infineon.com). Warnings Due to technical requirements, components may contain dangerous substances. For information on the types in question, please contact the nearest Infineon Technologies Office. Infineon Technologies components may be used in life-support devices or systems only with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered. Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Application Note AN190 Revision History: 15-02-2010 Previous Revision: Previous_Revision_Number Page Subjects (major changes since last revision) Application Note AN190, V1.0 3 / 15 15-02-2010 Low Barrier RF Schottky Diode BAT24-02LS Mixer for FMCW Radar at 24GHz Confidential List of Tables Table of Contents 1 Introduction ........................................................................................................................................5 2 2.1 2.2 RF Schottky Diode .............................................................................................................................6 Low Barrier RF Schottky Diode BAT24-02LS ......................................................................................7 Diode Mixer ..........................................................................................................................................9 3 3.1 3.2 Transfer Mixer used as Down Converter for FMCW Radar (24GHz) .............................................9 Design Concept of Transfer Mixer for RF Down Conversion...............................................................9 Simulation Results..............................................................................................................................11 List of Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Current status of worldwide frequency allocation for mm wave radar for automotive application .......5 Passive transfer single pole Mixer concept used for frequency down conversion ..............................6 Forward transmission characteristic of the Schottky diode..................................................................8 The serial resistance Rs causes at high currents (Ih) a voltage drop ∆U between the extrapolated straight line and the measured I(U) curve. The ideality factor n corresponds to the gradient of the IU-characteristic in forward operation and can be extracted within the linear region of the log(I(U)) diagram ................................................................................................................................................8 Transmitted and received signals of a Frequency Modulated Continuous Wave radar ......................9 Circuit for frequency down conversion from 24GHz to 200kHz achieved by the low barrier Schottky diode BAT24-02LS .............................................................................................................................10 PCB structures on RO3003................................................................................................................11 IF and RF spectral representation by applying the Harmonic Balance (HB) simulation....................12 Conversion loss and bias current over bias voltage are depicted .....................................................12 Conversion Gain in dependence of the incident LO power from the local oscillator. Sufficient LO amplitudes are needed in order to switch the RF signal on and off...................................................13 Conversion Gain in dependence of the received RF power from the antenna. The dynamic range is limited by the noise which was not included. .....................................................................................13 List of Tables Application Note AN190, V1.0 4 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential 1 Introduction Device: Low Barrier RF Schottky Diode BAT24-02LS Application: Mixer for FMCW Radar at 24GHz Automotive radar provides both a direct avoidance of collision and advanced driver assistance system functions like parking aid, blind spot surveillance or lane change support functions, respectively. The commercialization of automotive radar systems became feasible in the 90`s. All of these systems are operating above 17GHz whereas the most prominent radar applications are operating at 77GHz and 24GHz. This application note for mixer diode is focusing on automotive radar systems at 24GHz. Any device operating in this 24GHz ISM band must meet the existing regulations, which are given by the harmonized standards ETSI EN 302 288 in Europe [1]. Presently, the UWB (Ultra Wide Band) radar application at 24GHz frequencies is limited in time until 2013 except the narrow band at 24GHz. The UWB 24GHz radar system will be transferred to 77GHz (see Figure 1). Europe (ETSI) North America (FCC) Japan UWB SRR (Short Range Radar) Freely Available Freely Available 24GHz/26GHz Sunset Date in 2013 Narrowband Freely Available Freely Available Freely Available Freely Available Freely Available Freely Available Pending Pending Pending 24GHz LRR (Long Range Radar) 77GHz ACC UWB SRR 79GHz Figure 1 Current status of worldwide frequency allocation for mm wave radar for automotive application The complex and sometimes confusing but required regulations ensure a consistent and harmonious spectrum allocation which support sensor development without interfering with other systems. For the signal processing a frequency down conversion from 24GHz to the kHz range is needed in order to enable data processing of the transmitted and received signals like corresponding frequency or phase shifts of FMCW (Frequency Modulated Continuous Wave) radar systems or time delays for pulsed radar systems. This frequency down conversion can be realized by active mixers like Gilbert cells or passive mixers (e.g. diode mixers). Diode mixers can be applied over a remarkable range of frequencies and especially they are the best choice, if frequency conversion must be established in the mm wave range. This application note is presenting a low barrier Schottky diode BAT24-02LS from Infineon Technologies where the Si-die is housed in TSSLP-2-1 a very thin and small leadless package [2]. This device was especially processed for the high frequency range with low parasitic which guarantees a cost-effective, reliable, and flexible solution for the proposed single pole transfer mixer. However, this application is only suitable for FMCW Radar systems with mono-static antennas instead of bi-static antennas. The design concept for a single signal branch includes one Schottky diode together with PCB related RF structures like quarter wave transmission lines, RF stubs, and interdigital capacitors on a low loss RF substrate material. Application Note AN190, V1.0 5 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Figure 2 Passive transfer single pole Mixer concept used for frequency down conversion Figure 2 shows a simplified circuit consisting of a VCO (Voltage Controlled Oscillator) which generates for simplicity a linear chirp as shown in Figure 5. This generated LO (Local Oscillator) signal of the VCO is used in the mixer application as a pump signal in order to drive the nonlinear device in conductive and non-conductive state alternatively. After the VCO a 3dB power splitter is used in order to split the signal. For each single branch the pump signals are transmitted through the Schottky diode to the mono-static antennas. The mono-static antenna is used for both transmitting the LO signal and receiving the reflected LO signal from the object. The time delay during the pass to the target and back to the antenna is represented by a frequency shift of the FMCW signal (see also Figure 5). At least two antennas are required for calculating the position (distance and angle) of the target. 2 RF Schottky Diode The device characteristic of the Schottky diode is similar to a typical one sided abrupt pn diode which follows the same current voltage characteristic as being shown in equation (1). However, there are some magnificent differences between the pn junction diode and the Schottky diode. For example, the Schottky diode exhibits a lower forward voltage drop (0.15V to 0.45V) than the pn diode (0.7V to 1.7V). Furthermore, the voltage drop of Schottky diodes in forward direction can be adjusted by the applied contact material and also zero biased Schottky diodes can be processed based on p-doped materials. Moreover, pn junction diodes belong to minority semiconductor devices suffering on the recombination velocity of the minority carriers in the space charge region, whereas, the Schottky diodes are controlled by the charge transport over the barrier from the majority carriers. This leads to very fast switching action of the Schottky diodes and makes it very attractive for RF application in the mm wave range like mixers. qU d − 1 I = I S (T ) ⋅ exp nkT (1) (k: Boltzmann factor, n: ideality factor, IS: saturation current, Ud: voltage, T: temperature) 17 In normal forward operation at room temperature and moderate doping concentration (Nd < 10 following charge transports can be identified: Application Note AN190, V1.0 6 / 15 -3 cm ) the 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential − − − − Transport of electrons from semiconductor over the barrier to the metal Tunneling of electrons through the barrier Recombination in the space charge region Injection of holes from the metal to the semiconductor The ideality factor n corresponds to the gradient of the IU-characteristic in forward operation and can be extracted within the linear region of the log(I(U)) diagram as shown in Figure 4. Furthermore, the nonlinear behavior of the device corresponds to the fast switching from the conductive state to the non-conductive state by the LO signal. As the ideality factor n increases the nonlinearity of the device is reduced and the capability for frequency mixing is reduced as well. Therefore, for mixer application the ideality factor of the Schottky diode should be as small as possible, typically, smaller than 1.1. The voltage dependent junction capacitance Cj follows the equation (2) with the model parameter Uj which refers to the junction voltage and M as the grading coefficient (Μ = 0.5 for a uniformly doped diode). U C j (U d ) = C j 0 ⋅ 1 − d U j −M (2) Based on the small signal equivalent circuit the frequency conversion is also directly influenced by the serial resistance Rs and the junction capacitance Cj as shown in equation (3). Both Cj0 and Rs should be as small as possible and this characteristic figure of merit is represented by the cutoff frequency fc which should be as high as possible (3). The serial resistance Rs decreases and the junction capacitance Cj0 increases by increasing the device area A so that a first order analysis shows that the cut off frequency is independent of the junction area. However, second order effects reveal that the cut off frequency can be increased by decreasing the junction area. These and the non-linear junction capacitance Cj affect the mixer performance directly so that we have to optimize the Schottky diode appropriately in order to meet the mixer specification. 2.1 Low Barrier RF Schottky Diode BAT24-02LS If the diode is used in a circuit simulator, the diode is typically implemented by a spice netlist. Below, an extract of the spice model for the silicon die including package parasitic is shown for the product BAT24-02LS (basic type BAT24). ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ .SUBCKT BAT24_02LS 1 2 .MODEL Dmain D(IS=94.26nA N=1.039 Tt=0.1fs Cjo=106.3fF Vj=0.1 M=0.497 Fc=.5 Msw=0.33 Fcsw=0.5 Xti=2.5 Eg=0.69 Tnom=25) .Model Dsat D(Is=60mA N=0.5 Cjo=1f Xti=2.5 Eg=0.69 Tnom=25) .Model Dlow D(Is=1pA N=2 Xti=2.5 Eg=0.69 Tnom=25) .Model Drev D(Is=12.9uA N=45 Cjo=1fF Xti=2.5 Eg=0.69 Tnom=25) Rs1 21 22 2.00 Rs2 1 10 3.50 Ls2 10 11 1.80e-10 Ls1 11 12 1.15e-10 Cp11 11 31 7.45e-14 Rp11 31 2 3.43 Cp2 1 2 2.40e-14 Rp1 11 2 6.13e+7 Dmain 22 2 Dmain Drev 2 12 Drev Dsat 12 21 Dsat Dlow 21 2 Dlow .ENDS BAT24_02LS ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Application Note AN190, V1.0 7 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential The dc characteristics of the diode are determined by the saturation current IS and the ideality factor N (n) which also represents the nonlinear characteristic of the device. The distributed bulk resistance of Rs1+Rs2=5.5Ω is included which describes the IU-characteristic of the device beyond 400mV which is leading to current limitation. This can easily be seen by replacing the internal applied voltage Ud by Uext – I · RS in equation (1) whereas Uext refers to the external applied voltage. Charge storage effects are modeled by the transit time, TT, and a nonlinear depletion layer capacitance which is determined by the parameters CJO, VJ, and M. The temperature dependence of the saturation current is defined by the parameters EG, the activation energy and XTI, the saturation current temperature exponent. The nominal temperature at which these parameters were measured is TNOM=25°C. The diode is housed in an extremely low parasitic TSSLP-2-1 package with small variation on wire bond length and wire bond shape. The RF Schottky diode was optimized regarding junction capacitance and the serial wire bond inductance so that a serial resonance occurs at 24GHz as being shown in Figure 3. Figure 3 Forward transmission characteristic of the Schottky diode The nonlinear voltage current characteristic is depicted in Figure 4. Figure 4 The serial resistance Rs causes at high currents (Ih) a voltage drop ∆U between the extrapolated straight line and the measured I(U) curve. The ideality factor n corresponds to the gradient of the IU-characteristic in forward operation and can be extracted within the linear region of the log(I(U)) diagram Application Note AN190, V1.0 8 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential 2.2 Diode Mixer The harmonic balance (HB) method is typically used for the simulation and optimization of mixer circuits. The circuit is split into a nonlinear sub-circuit and linear sub-circuit and the large signal solution is iteratively found by numerical methods and consists of the harmonics for the current and voltage waveforms [3]. Figure of Merits for BAT24-02LS a) Cutoff frequency: 230GHz ωc = 1 Rs C j 0 (3) b) Conversion Loss: 9dB Lc , dB = Pin , dB ( f rf ) − Pout , dB ( f if ) 3 (4) Transfer Mixer used as Down Converter for FMCW Radar (24GHz) This application is focusing on FMCW Radar. The instantaneous difference between the transmitted and received frequencies, ∆f is measured as shown in Figure 5. This difference is direct proportional to the time delay ∆t of the radar signal to reach the target and return back to the mono-static antenna. Afterwards the distance from the antenna to the target can be calculated by the frequency shift. Figure 5 Transmitted and received signals of a Frequency Modulated Continuous Wave radar 3.1 Design Concept of Transfer Mixer for RF Down Conversion For simplicity, the transfer characteristic of the incident and reflected waves from antenna to the target and vice versa is replaced by a second RF signal generator operating at frf = fIF + flo and a circulator (only used for the simulation). The other devices can easily be realized by standard PCB technology (e.g. RO3003 from Rogers). The circuit is now described at DC (0Hz), IF (200kHz), and RF (24.0002GHz) / LO (24GHz): − at DC: the correct biasing of the diode can be accomplished by a serial resistance of 2.75kOhm together with a dc voltage source of 4V. The current loop is closed over the transmission line DA_Mline2. − at IF: die RF signal is down converted to the intermediate frequency (IF). The ohmic IF source impedance Zif is not transformed over the DA_Mline2 (frf/fif = 120000) and therefore directly connected on the diode. Application Note AN190, V1.0 9 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Moreover, the RF butterfly structure (shunted capacitor) is only a short for the LO and RF signals whereas the IF signal is not affected by this ground. − At RF and LO: all shorts are transformed by the λ/4 lines into an open so that this signals are directly transferred to the RF source and back. The interdigital capacitor is used for dc blocking and can also be realized on PCB. The presented solution combines the benefits of PCB concept together with the RF Schottky diode so that a very cost effective and flexible solution can be offered. This approach requires no isolation between the RF and LO sources and is therefore different to other concepts where hybrids or filters are used in order to achieve high isolation between the RF and IF sources. Figure 6 Circuit for frequency down conversion from 24GHz to 200kHz achieved by the low barrier Schottky diode BAT24-02LS Application Note AN190, V1.0 10 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Figure 7 PCB structures on RO3003 3.2 Simulation Results The harmonic balance simulation was carried out under ADS2008 with following constraints: − Transmitted RF power of 3dBm (PLO) from the local oscillator at 24GHz − Received RF power of -30dBm (Prf) from antenna which corresponds to the RF source at 24.0002GHz The simulation results are shown in the following figures: In Figure 8 the RF and IF spectra are shown by applying a LO power of 3dBm. The RF power at 24.0002GHz is frequency down-converted to the intermediate frequency fif. The passive down-conversion exhibits a conversion loss Lc of about 9dB. After Figure 8 also additional harmonics at fnm = n · flo ± m · frf are generated whereas the intermediate frequency corresponds to fif = f(-1,1) = 200kHz. The IF-power refers to the impedance Zif of 50Ohm. Additional filter structures can be used in order to suppress unwanted harmonics. Application Note AN190, V1.0 11 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Figure 8 IF and RF spectral representation by applying the Harmonic Balance (HB) simulation The capability of frequency conversion of the diode is represented by the conversion loss Lc.after (4). In Figure 9 the conversion loss and dc bias current over bias voltage are shown. Proper biasing can improve the conversion loss. For that we assumed a dc bias voltage of 4V which determines the required serial resistance of 2750 Ohm. Figure 9 Conversion loss and bias current over bias voltage are depicted In the following Figure 10 the conversion loss over PLO is shown. The LO signal (pump signal) must drive the nonlinear device in strong conductive and strong non-conductive states alternatively. Therefore, the LO signal must be higher than 3dBm but lower that 25dBm so that a conversion loss of lower than 10dB can be obtained. Application Note AN190, V1.0 12 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Figure 10 Conversion Gain in dependence of the incident LO power from the local oscillator. Sufficient LO amplitudes are needed in order to switch the RF signal on and off Figure 11 shows the conversion loss over the RF input power. Because of neglecting noise in the simulation the conversion loss is not restricted towards very low RF input power. At RF input power larger than -6dBm (1dB compression point) the linear down-conversion is not anymore guaranteed so that the frequency conversion ends in compression. Figure 11 Conversion Loss in dependence of the received RF power from the antenna. The dynamic range is limited by the noise which was not included. Application Note AN190, V1.0 13 / 15 15-02-2010 Application Note AN190 Mixer for FMCW Radar at 24GHz Confidential Links and References [1] http://etsi.org [2] Datasheet - BAT24: http://www.infineon.com/cms/en/product/ [3] Stephen A. Maas, “Microwave Mixers”, Artech House, Boston - London, 1993 Application Note AN190, V1.0 14 / 15 15-02-2010 w w w . i n f i n e o n . c o m Published by Infineon Technologies AG AN190